In the manufacture of paper an aqueous cellulosic suspension or slurry is formed into a paper sheet. The cellulosic slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1 percent. Often a slurry of below 0.5 percent is used just ahead of the paper machine. However, while the finished sheet must have less than about 6 weight percent water. Hence the dewatering aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
The least costly dewatering method is simple drainage. More expensive methods which are used include vacuum, pressing, felt blanket blotting and pressing, and evaporation. In practice a combination of such methods are employed to dewater, or dry the sheet to the desired water content. Since drainage is both the first dewatering method employed and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost of the manufacture is retention of furnish components on and within the fiber mat being formed during papermaking. A paper making furnish generally contains particles that range in size from the 2 to 3 millimeters of cellulosic fibers, to fillers at a few microns and to colloids. Within this range are cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would in significant portion pass through the spaces (pores) between the cellulosic fibers in the fiber mat being formed during papermaking.
One method of improving the retention of cellulosic fines, mineral fillers, and other furnish components on the fiber mat is the use of a coagulant/flocculant system added ahead of the paper machine. In such a system there is first added a coagulant, for instance a low molecular weight synthetic cationic polymer or a cationically modified starch to the furnish, which coagulant generally reduces the negative surface charges present on the particles in the furnish, particularly cellulosic fines and mineral fillers, and thereby accomplishes a degree of agglomeration of such particles, followed by the addition of a flocculant. Such flocculant generally is a high molecular weight anionic synthetic polymer which bridges the particles and/or agglomerates, from one surface to another, binding the particles into large agglomerates. The presence of such large agglomerates in the furnish as the fiber mat of the paper sheet is being formed increases the retention of particles to the fiber mat. The agglomerates are filtered out of the water onto the fiber web where unagglomerated particles would to a great extent pass through such paper web.
While a flocculated agglomerated generally does not interfere with the drainage of the fiber mat to the extent that would occur if the furnish were gelled or contained an amount of gelatinous material, when such flocs are filtered by the fiber web the pores thereof are to a degree reduces, reducing the drainage efficiency therefrom. Hence the retention is being increased with some degree of deleterious effect on the drainage.
Another system employed to provide an improved combination of retention and dewatering is described in U.S. Pat. Nos. 4,753,710 and 4,913,775, inventors Langeley et al., both of which are hereinafter incorporated by reference. In brief, such method adds to the aqueous cellulosic papermaking suspension first a high molecular weight linear cationic polymer followed by the addition of bentonite after shearing. The shearing generally is provided by one or more stages of the papermaking process and the shearing breaks down the large flocs formed by the high molecular weight polymer into microflocs, and further agglomeration then ensues with the addition of the bentonite particles.
Another system uses the combination of cationic starch followed by colloidal silica to increase the amount of material retained on the web by the method of charge neutralization and adsorption of smaller agglomerates. This system is described in U.S. Pat. No. 4,388,150. Yet another variation of this system is described in U.S. Pat. Nos. 4,643,801 and 4,750,974, both of which are hereinafter incorporated by reference which in addition to the use of a cationic starch and colloidal silica employ, with or without the starch, a high molecular weight anionic polymer.
U.S. Pat. No. 4,795,531 teaches the use of a retention and drainage aid program consisting of a low molecular weight cationic polymer coagulant, colloidal silica sol and a high molecular weight polymer flocculant which may be anionically or cationically charged.
Additional systems to improve drainage and retention have also been proposed. Among these systems are the use of a single, high molecular weight cationic polymer as exemplified in South African Patent 2389/90 corresponding to U.S. Ser. No. 397,224 filed Aug. 23, 1989. U.S. Pat. No. 5,098,520 suggests a drainage and retention program in which, a cellulosic papermaking slurry containing a mineral filler is treated with a high molecular weight cationic (meth)acrylamide polymer prior to at least one shear stage followed by the addition of a low molecular weight anionic polymer at least one shear stage subsequent to the addition of the cationic polymer.
Dewatering generally, and particularly dewatering by drainage, is believed improved when the pores of the paper web are less plugged, and it is believed that retention by adsorption in comparison to retention by filtration reduces such pore plugging.
Greater retention of fines and fillers permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of less quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes even more important because the fines content of such lower quality pulps is greater generally than that of pulps of higher quality.
Greater retention of fines, fillers, and other slurry components reduces the amount of such substances lost to the white water and hence reduces the amount of material wastes, the cost of waste treatment and disposal, and the adverse environmental effects therefrom.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation is determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90 percent, the formation parameter generally abruptly declines from good formation to poor formation. It is at least theoretically believed that as the retention mechanisms of a given papermaking process shift from filtration to adsorption, the deleterious effect on formation, as high retention levels are achieved, will diminish and a good combination of high retention with good formation is attributed to the use of bentonite in U.S. Pat. No. 4,913,775.
It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
It is also desirable to use additives that can be delivered to the paper machine without undue problems. Additives that are easily dissolved or dispersed in water minimize the expense and energy required for delivering them to the paper machine and provide a more reliable uniformity of feed than additives which are not easily dissolved or dispersed.